Electrolytic condenser and method of making same



March 28, 1939. N. SCHNOLL ET! AL 2,151,306

ELECTROLYTIC CONDENSER AND METHOD OF MAKING SAME Filed June 5, 1957 I? I5 I INVENTOR. NaMd/Y Schnall Lewis WA lefon $3 LE? ATT R 8,

Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE ELECTROLYTIC CONDENSER AND METHOD OF MAKING SAME Nathan Schnoll, New York, and'Lewis W. Appleton, Brooklyn, N. Y.,

assignors to Solar Manu- Application June 5, 1937, Serial No. 146,540

3 Claims.

This invention relates to improvements in electrolytic condensers both of the wet and drytype, and in the methods of testing and manufacturing them.

A general object of this invention is to provide an improved form of electrolytic condenser of either the wet or dry type in that for a given capacity and peak voltage the condenser of this invention is substantially smaller in size than those heretofore known of the same capacity and peak voltage.

Another object of this invention is to provide electrolytic condensers which are at least 50% smaller in overall size or in the volume of electrolyte than standard condensers of the same type heretofore known of the same capacity and peak voltage rating.

A further object of this invention is to provide an electrolytic condenser having a very low leakage current after long idling periods.

Another object of this invention is to provide electrolytic condensers having anode films which do not deteriorate appreciably after long idling periods.

A further object of this invention is to produce electrolytic condensers having greatly improved idling characteristics.

A further object of this invention is the provision of electrolytic condensers having anodes which are more stable under continuous applied voltages at high temperatures and under alternating currents than has heretofore been known.

A further object of this invention is the provision of a method of testing electrolytic condensers to determine the deterioration characteristics thereof.

A further object of this invention is the provision of methods of hastening the deterioration of the anode film to improve the idling characteristics thereof.

These and many other objects, as will appear from the following disclosure, are secured by means of this invention.

This invention resides substantially in the combination, construction, arrangement, relative location of parts, stepsand series of steps, all in accordance with the following disclosure.

In the accompanying drawing,

Figure 1 is a vertical, cross-sectional view taken on the line l-l of Fig. 2. illustrating a wet electrolytic condenser in accordance with this inve tion;

Fig. 2 is a cross-sectional view taken on the line 2-2 of Figure 1;

Fig.3isaviewsimilartothatofli'lgurel ninety degrees displaced, showing some parts in elevation;

Fig. 4 is a top plan view of the condenser;

Figs. 5 and '7 are side elevational views of a standard condenser and a condenser in accord- 5 ance with this invention of the same rating, illustrating the relative sizes thereof; and

Figs. 6 and 8 are top plan views of the condensers of Figs. 5 and 7 respectively.

For a full understanding of the nature and 10 scope of this invention, the idling characteristics of electrolytic condensers, both of the wet and dry types, may be defined as the changes in electrical properties (capacity, power-factor and leakage current) resulting from periods of non- 16 use. These changes are detected by the application of a measuring voltage to the condenser at the termination of the period of non-use. In accordance with this invention electrolytic condensersare treated at the time of manufacture 20 so as to have low leakage currents and low power factors, these values remaining substantially constant even after long periods of non-use.

It is common practice today to immerse them in a suitable electrolyte such as water, borax and 25 boric acid and connect them to the positive terminal of a direct current source, the negative terminal of which is connected to the electrolyte.

A current of about 3 milliamperes per square inch of total anode surface is passed through the electrode and maintained at a constant value as the voltage across the cell rises and the film forms. When the voltage rises to the maximum desired value of say for example 500 volts it is maintained constant, while the current is permitted to drop. When this leakage current decreases to a sufiiciently low value, such as 0.1 milliampere per square inch, the formation is complete and the anode is removed from the bath.

During the film formation the temperature of the electrolyte is generally maintained at about degrees centigrade. Electrolytic condensers made from anodes thus formed have heretofore been described as satisfactory as far as initial electrical characteristics are concerned. An inherent undesired characteristic of anodes thus formed is that the film on the surface of the aluminum gradually loses its insulating properties when the condenser made therefrom is idled with no voltage applied for periods extending 50 over several days. While the anode films of such condensers do not all deteriorate or lose their insulating properties to the same extent, it may be stated as a general fact that the deterioration in all is undesirably great.

We have found that this deterioration of the anode nlm occurs more rapidly when the condensers are stored at elevated temperatures. For example. in some cases we have found that the deterioration after four hours of idling at degrees centigrade appears to be equivalent to several months idling at 20 degrees centigrade. Upon the basis of this discovery we have provided a practical method of forecasting the idling characteristics of electrolytic condensers. A number of completed condensers are first taken from a given batch and placed in an oven at an elevated temperature of say for example 85 degrees centigrade for a period oi several hours, as for example four hours. At the end oi this treatment the condensers are tested for leakage current characteristics, and upon the basis of the data thus gathered it is possible to determine the idling characteristics of the remainder of the batch, that is the extent to which the anode film would deteriorate at normal temperatures after a long idling period such as for example several months. The following table contains the test data of a number of condensers from several batches after heat treatment in accordance with the above process when a test voltage of 500 volts is applied thereto- Leakaae current-milliamperes Initial 15 sec. 30 sec. 1 min.

We have found that condenser A will deteriorate more rapidly than condenser B when idled for long periods of time at normal temperatures, and that condensers C and D will show very little deterioration even after long periods of idling. Thus by this method of testing condensers the idling characteristics oi the remainder of the batch from which they were taken can be readily forecast, which is of great practical advantage.

We have discovered further, that condensers having idling characteristics similar to A or B of the above tabulation do not deteriorate to the same extent the next time the condensers are idled for the same length of time under the same conditions. This is illustrated by the following results showing the leakage measurements obtained by applying rated voltage to a condenser, after each of five successive idling periods at 85 degrees centigrade. The idling period in each case was of four hours duration.

Leakage current Initial 15 sec. 30 sec.

After 1st heat cycle 110 58 45 After 2nd heat cycle 110 35 22 After 3rd heat cycle 1a 10 Ai'ter 4th heat cycle 00 13 0 After 5th heat cycle 60 i0 8 It is within the scope of this invention to accomplish the same result by idling the condensers at room temperature for a number of cycles each followed by the application of full voltage. The apparent disadvantage of this process is the relatively long time necessary to complete the treatment. If the condensers are idled at normal temperatures instead of elevated temperatures,

and after each period of idling are subjected to full voltage, the improvements herein disclosed may be secured, but obviously as a matter of convenience, it is preferable to shorten the idlina periods by the heat treatment explained above.

From a commercial manufacturing viewpoint the following method of improving the idling characteristics of electrolytic condensers is pre ferred. The anodes are prepared for the formation of film thereon in the regular way and then formed in the regular standard forming solutions in accordance with standard practice. To insure a complete disclosure it may be noted, as one example of standard practice, that the forming solution consists of 12% boric acid and 0.15% borax, by weight, dissolved in water. A batch of anodes are immersed therein connected to the positive terminal of a D. C. source, the negative terminal of which is connected to the electrolyte. A current of about 3 milliamperes per square inch of total anode surface is passed through the electrodes and maintained at a constant value while the voltage rises as the film forms. when the voltage reaches a desired maximum value, such as 500 volts, it is maintained constant while the leakage current is permitted to drop until it reaches a sufliciently low value such as 0.1 milliampere per square inch. The bath is maintained at a temperature close to 100 degrees centi rade.

The formed anodes are then treated as follows: They are permitted to idle in the hot formation solution, that is without any voltage applied thereto for about five minutes, the temperature of the solution being maintained at approximately 100 degrees centigrade. The direct current voltage previously employed, that is 500 volts, is again applied until the leakage current falls to about 2 milliamperes per square inch The anodes are then idled for about thirty minutes and the voltage again applied until the leakage current falls to 0.1' milliampere per square inch.

The anodes are then subjected to two idling periods of one hour each, followed in each casev of course, by the application of the full voltage so that the final leakage current falls to the desired low value, as for example 0.1 milliamperc per square inch of total anode surface.

It is then desirable, but not necessary, to give the anodes two additional idling period treat ments of four hours each at a temperature of about 100 degrees centigrade as in each of the previous cases, followed by the re-formation of the film by the application of direct current voltage thereto.

Further idling periods and subsequent re-fcrmation by reapplication of voltage may be carried out until the film is as stable as desired.

For all practical purposes we have not found 20 acid and 3% of borax dissolved in water.

it necessary to proceed beyond the two four hour periods mentioned in the previous paragraph.

After this treatment the anodes are ready for assembly in condensers and the film thereon will I 5 withstand long idling periods at room temperatures without material increase in the leakage current. We do not know clearly why this improvement is secured, although we believe that it involves some change in the structure of the i film so that it is much less subject to deteriora- 15 idling periods may be shortened by employing a different solution in which they are idled. This second solution which we employ, because it has been found that the film deteriorates more rapidly therein, may for example consist of 12% boric The film is deteriorated in the solution which has the higher borax content but is re-formed each time in the original forming solution. Otherwise the process of treatment is the same. It will be apparent to those skilled in the art that the number of idling periods can be varied depending upon how stable it is desired the ultimate film is to be. Other mediums may be substituted for the forming solution during the idling period 30 such as moist air or distilled water.

Another very important improvement in the art arises as a result of the above treatment.

The physical size of electrolytic condensers is, to a large degree, determined by its heat dissipating ability. After a condenser has idled for a period of time so that its leakage current has materially increased when it is first placed in circuit a considerable amount of heat is generated as a result of this leakage current which 40 the condenser must be capable of dissipating without damage. It is immediately apparent,

therefore, that when anodes are treated in accordance with the processes herein disclosed, with the result that their leakage current re- 45 mains low after long periods of idling, they may be made much smaller than heretofore because of the decrease in the amount of heat that must be dissipated when first put back in circuit after idling.

The decrease in the size of the condenser possible as a result of this invention is so substantial as to be of great practical importance. We have graphically shown in Figs. 5 to 8 inclusive the decrease in size made possible. In Figs. 5

5 and 7 are shown two condensers of the same 65 condenser of Figs. 7 and 8 by reason of the fact that its low leakage current remains substantially -constant for all normal idling periods likely to be encountered, need not dissipate as much heat when put back on the line as would be true of 70 the condenser of Figs. 5 and 6. To show the remarkable reduction in size gained, it is noted that the condenser of Fig. 5 contains about 40 c. c. of electrolytewhile the condenser of Fig. 7 contains but 12 c. c. of electrolyte. It is not un- 7 usual to find at the present time in commercial use approved electrolytic condensers of the same capacity and peak voltage as those of Figs. 5 and 7, which contain as high as 75 c. c. of elec-,- trolyte. It seems clear, therefore, that the condenser of Fig. 7 represents a great improvement in the art by reason of its reduction in size andccst and the facility with which it may be employed in small spaces.

Another important improvement resides in the fact that anodes with the film treated as described will operate satisfactorily for longer periods of time at elevated temperatures and high D. C. voltage than ones not so treated. We have discovered that such treated anodes are more stable with A. C. voltage applied than normal units under similar conditions.

We have illustrated in Figs. 1 to 4 inclusive one form of wet electrolytic condenser made in accordance with the invention herein disclosed. It comprises a metal container i preferably of aluminum having formed integral therewith a hollow external threaded hub 2 tapered on the inside to receive an insulating bushing 9 in which is mounted a stud comprising a conical portion 8 and a cylindrical portion 1. Riveted to the upper end of the stud by the head ll is a supporting bar l5 which is clamped between the head of the rivet and a washer "resting on the end of the bushing. The lower end of the stud projects through the lower end of the bushing, a washer ill, and the hub ii on the terminal clip i2. The parts are compressed so as to place the bushing 9, which is for example of some compressible material such as rubber, and the hub ii is compressed under considerable pressure onto the stud to lock the parts together and to form a fiat tight joint.

At I8 is shown the anode which has a film formed thereon in accordance with this invention, which anode is provided with a wing i6 riveted to the supporting arm I5 at the points II. This arm supports the anode within the container and spaced therefrom. The upper end of the container is closed by means of a bead 3 which fits over a disc 5 which has a rolled edge as shown at 5', and which with the bead 3 clamps in fluid tight relation a rubber gasket 6 which seals the entire end of the container. A spacing member 4 which comprises a disc having a central hub is mounted on the disc 5 in the manner shown and engaged at the boss by the gasket. The boss and disc are provided with a small vent hole. This construction is in accordance with one commercial form of device with the exception of the treated anode film. This condenser is disclosed herein in the detail given to provide a basis for -claims for the improved condenser of this invention.

We are well aware that those skilled in the ,art will immediately appreciate variations in the in vention herein disclosed, both in structure and procedure, without departing, however, from the true scope of the invention. We do not, therefore, desire to be strictly limited to the disclosure as given for purposes of illustration, but rather to the scope of the appended claims.

What is claimed is:

1. A method of manufacturing anodes for electrolytic condensers, which comprises forming a film on the anode in a boric acid and borax forming solution, immersing the anode in a similar solution having a higher borax conteni at an elevated temperature for a. predetermined period of time to deteriorate the film, again immersing the anode in said forming solution, ap-

ill

plying a direct current voltage to the anode to re-i'orm the film, and repeating the steps of deteriorating the film and re-forming it.

2. In the manufacture of an electrode for electrolytic condensers, the steps oi forming an oxide film on an electrode in a heated electrolyte solutlon until the leakage current of the formed film in the electrolyte has iallen to less than 0.5 milliampere per square inch of film, idling the filmed electrode in said electrolyte for a plurality of time intervals to deteriorate the film, the first interval being longer than fifteen minutes and a later interval being longer than the first, and apiying a direct current voltage to the electrode while immersed in said electrolyte after each idling interval until the final leakage current of the film is of the order of 0.1 milliampere per square inch of film.

3. An electrolytic condenser including a filmed electrode and an electrolyte having a leakage current oi the order of 0.1 milliampere per square inch of film, said leakage current remaining substantially constant over long idling periods, and the volume of electrolyte being less than two cubic centimeters per microfaraci at 500 volts, said filmed electrode having been subjected to the steps of forming an oxide film thereon in a heated electrolyte solution until the leakage current of the formed film in the electrolyte has fallen to less than 0.5 milliampere per square inch 01' film, idling the filmed electrode in said electrolyte for a plurality 01' time intervals to deteriorate the film, the first interval being longer than fifteen minutes and a later interval being longer than the first, and applying a direct current volt age to the electrode while immersed in said electrolyte after each idling period until the final leakage current is of the order of 0.1 milliampere per square inch of film.

LEWIS W. APPLETON. NATHAN BCHNOLL. 

